Integrated interface electronics for reconfigurable sensor array

ABSTRACT

An integrated switch matrix for reconfiguring subelements of a mosaic sensor array to form elements. The configuration of the switch matrix is fully programmable. The switch matrix includes access switches that connect subelements to bus lines and matrix switches that connect subelements to subelements. Each subelement has a unit switch cell comprising at least one access switch, at least one matrix switch, a respective memory element for storing the future state of each switch, and a respective control circuit for each switch. The access and matrix switches are of a type having the ability to memorize control data representing the current switch state of the switch, which control data includes a data bit input to turn-on/off circuits incorporated in the control circuit. The sensor array and the switching matrix may be built in different strata of a co-integrated structure or they may be built on separate wafers that are electrically connected. If the sensors are arranged on a hexagonal grid, the unit switch cells may be arranged on either a hexagonal or rectangular grid.

RELATED PATENT APPLICATION

This application is a continuation-in-part of and claims priority fromU.S. patent application Ser. No. 10/383,990 filed on Mar. 6, 2003 andentitled “Mosaic Arrays Using Micromachined Ultrasound Transducers”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

The United States Government may have certain rights in this inventionpursuant to U.S. Government Contract Number DAMD17-02-1-0181 awarded bythe U.S. Army.

BACKGROUND OF THE INVENTION

This invention generally relates to reconfigurable arrays of sensors(e.g., optical, thermal, pressure, ultrasonic). In particular, theinvention relates to reconfigurable micromachined ultrasonic transducer(MUT) arrays. One specific application for MUTs is in medical diagnosticultrasound imaging systems. Another specific example is fornon-destructive evaluation (NDE) of materials, such as castings,forgings, or pipelines.

Conventional ultrasound imaging systems comprise an array of ultrasonictransducers that are used to transmit an ultrasound beam and thenreceive the reflected beam from the object being studied. Such scanningcomprises a series of measurements in which the focused ultrasonic waveis transmitted, the system switches to receive mode after a short timeinterval, and the reflected ultrasonic wave is received, beamformed andprocessed for display. Typically, transmission and reception are focusedin the same direction during each measurement to acquire data from aseries of points along an acoustic beam or scan line. The receiver iscontinuously refocused along the scan line as the reflected ultrasonicwaves are received.

For ultrasound imaging, the array typically has a multiplicity oftransducers arranged in one or more rows and driven with separatevoltages in transmit. By selecting the time delay (or phase) andamplitude of the applied voltages, the individual transducers in a givenrow can be controlled to produce ultrasonic waves that combine to form anet ultrasonic wave that travels along a preferred vector direction andis focused in a selected zone along the beam.

The same principles apply when the transducer probe is employed toreceive the reflected sound in a receive mode. The voltages produced atthe receiving transducers are summed so that the net signal isindicative of the ultrasound reflected from a single focal zone in theobject. As with the transmission mode, this focused reception of theultrasonic energy is achieved by imparting separate time delay (and/orphase shifts) and gains to the signal from each receiving transducer.The time delays are adjusted with increasing depth of the returnedsignal to provide dynamic focusing on receive.

The quality or resolution of the image formed is partly a function ofthe number of transducers that respectively constitute the transmit andreceive apertures of the transducer array. Accordingly, to achieve highimage quality, a large number of transducers is desirable for both two-and three-dimensional imaging applications. The ultrasound transducersare typically located in a hand-held transducer probe that is connectedby a flexible cable to an electronics unit that processes the transducersignals and generates ultrasound images. The transducer probe may carryboth ultrasound transmit circuitry and ultrasound receive circuitry.

A reconfigurable ultrasound array is one that allows groups ofsubelements to be connected together dynamically so that the shape ofthe resulting element can be made to match the shape of the wave front.This can lead to improved performance and/or reduced channel count.Reconfigurability can be achieved using a switching network.

Recently semiconductor processes have been used to manufactureultrasonic transducers of a type known as micromachined ultrasonictransducers (MUTs), which may be of the capacitive (MUT) orpiezoelectric (pMUT) variety. MUTs are tiny diaphragm-like devices withelectrodes that convert the sound vibration of a received ultrasoundsignal into a modulated capacitance. For transmission the capacitivecharge is modulated to vibrate the diaphragm of the device and therebytransmit a sound wave. One advantage of MUTs is that they can be madeusing semiconductor fabrication processes, such as microfabricationprocesses grouped under the heading “micromachining”. The systemsresulting from such micromachining processes are typically referred toas “micromachined electromechanical systems (MEMS). As explained in U.S.Pat. No. 6,359,367:

-   -   Micromachining is the formation of microscopic structures using        a combination or subset of (A) Patterning tools (generally        lithography such as projection-aligners or wafer-steppers),        and (B) Deposition tools such as PVD (physical vapor        deposition), CVD (chemical vapor deposition), LPCVD        (low-pressure chemical vapor deposition), PECVD (plasma chemical        vapor deposition), and (C) Etching tools such as wet-chemical        etching, plasma-etching, ion-milling, sputter-etching or        laser-etching. Micromachining is typically performed on        substrates or wafers made of silicon, glass, sapphire or        ceramic. Such substrates or wafers are generally very flat and        smooth and have lateral dimensions in inches. They are usually        processed as groups in cassettes as they travel from process        tool to process tool. Each substrate can advantageously (but not        necessarily) incorporate numerous copies of the product. There        are two generic types of micromachining . . . 1) Bulk        micromachining wherein the wafer or substrate has large portions        of its thickness sculptured, and 2) Surface micromachining        wherein the sculpturing is generally limited to the surface, and        particularly to thin deposited films on the surface. The        micromachining definition used herein includes the use of        conventional or known micromachinable materials including        silicon, sapphire, glass materials of all types, polymers (such        as polyimide), polysilicon, silicon nitride, silicon oxynitride,        thin film metals such as aluminum alloys, copper alloys and        tungsten, spin-on-glasses (SOGs), implantable or diffused        dopants and grown films such as silicon oxides and nitrides.        The same definition of micromachining is adopted herein.

The cMUTs are usually hexagonal-shaped structures that have a membranestretched across them. This membrane is held close to the substratesurface by an applied bias voltage. By applying an oscillatory signal tothe already biased cMUT, the membrane can be made to vibrate, thusallowing it to radiate acoustical energy. Likewise, when acoustic wavesare incident on the membrane the resulting vibrations can be detected asvoltage changes on the cMUT. A cMUT cell is the term used to describe asingle one of these hexagonal “drum” structures. The cMUT cells can bevery small structures. Typical cell dimensions are 25-50 microns fromflat edge to flat edge on the hexagon. The dimensions of the cells arein many ways dictated by the designed acoustical response. It may not bepossible to create larger cells that still perform well in terms offrequency response and sensitivity desired.

Unfortunately, it is difficult to produce electronics that would allowindividual control over such small cells. While in terms of theacoustical performance of the array as whole, the small cell size isexcellent and leads to great flexibility, control is limited to largerstructures. Grouping together multiple cells and connecting themelectrically allows one to create a larger subelement, which can havethe individual control while maintaining the desired acousticalresponse. So a subelement is a group of electrically connected cellsthat cannot be reconfigured. For the purpose of this disclosure, thesubelement is the smallest independently controlled acoustical unit. Onecan form rings or elements by connecting subelements together using aswitching network. The elements can be reconfigured by changing thestate of the switching network. However, subelements comprise connectedcells that are not switchably disconnectable and thus cannot bereconfigured. All of the following analysis is also valid if the arrayis made of PZT or some other more common or future transducertechnology.

Reconfigurability using silicon-based ultrasound transducer subelementswas described in U.S. patent application Ser. No. 10/383,990. One formof reconfigurability is the mosaic annular array, also described in thatpatent application. The mosaic annular array concept involves buildingannular elements by grouping subelements together using a reconfigurableelectronic switching network. The goal is to reduce the number ofbeamforming channels, while maintaining image quality and improvingslice thickness. To reduce system channels, the mosaic annular arraymakes use of the fact that for an unsteered beam, the delay contours onthe surface of the underlying two-dimensional transducer array arecircular. In other words, the iso-delay curves are annuli about thecenter of the beam. The circular symmetry of the delays leads to theobvious grouping of those subelements with common delays and leads tothe annular array concept. The reconfigurability can be used to step thebeam along the larger underlying two-dimensional transducer array inorder to form a scan or image. The reconfigurability might also be usedto improve performance for multiple transmit applications by assigningmore channels to the smaller active aperture in the near field. Thereare many other applications where reconfigurability might prove useful.

In a mosaic annular transducer array and other mosaic transducer arrays,a large number of ultrasound transducer subelements must be connectedtogether using a distributed switch matrix. The subelements build uplarger elements that are used for transmission and reception ofultrasound signals. The configuration of the elements and therefore thesubelements changes each time that a new line of data or “view” isacquired. Each time that the configuration changes, the state (on oroff) of all of the switches in the switching matrix must be updated tocreate the required interconnections that build up the new state of theelements and subelements.

In a reconfigurable sensor array, a large number of sensor subelementsmust be accessed by system electronics. This presents a significantbottleneck in terms of routing of signal and control lines to associatedsystem processing electronics.

In current high-channel-count systems, connections to individual sensorelements are brought out using individual flexible wires and routed toexternal printed circuit boards housing the necessary scanningelectronics. The wiring and printed circuit boards are bulky and notcurrently applicable to a very large number of transducer subelements asis the case in a mosaic transducer array.

Reconfigurable ultrasound arrays require a complex switching networkthat may be difficult or impossible to implement with currentlyavailable electronics. There is a need for a simplified switchingnetwork that has application in arrays of ultrasonic transducersubelements as well as in arrays of other types of sensors (e.g.,optical, thermal, pressure). There is also a need for a constructioncomprising integrated switching electronics disposed beneath the sensorarray for rapidly reconfiguring the sensor array.

BRIEF DESCRIPTION OF THE INVENTION

The invention is directed to an integrated switch matrix forreconfiguring subelements of a mosaic sensor array. The configuration ofthe switch matrix is fully programmable. The switch matrix includesaccess switches that connect subelements to bus lines and matrixswitches that connect subelements to subelements. Each subelement has aunit switch cell associated therewith. In one embodiment, each unitswitch cell comprises at least one access switch, at least one matrixswitch, a respective pair of latches for storing the future state ofeach switch, and a respective control circuit for each switch. Theaccess and matrix switches are of a type having the ability to memorizecontrol data representing the current switch state of the switch, whichcontrol data includes one data bit input to turn-on/off and turn-offcircuits incorporated in the control circuit. The sensors may beoptical, thermal or pressure sensors or ultrasonic transducers. Thesensor array and the switching matrix may be built in different strataof a co-integrated structure or they may be built on separate wafersthat are electrically connected. It may also be possible to build thesensors in the same strata as the electronics; however, this woulddecrease the amount of area available to the electronics and istherefore less desirable. If the sensors are arranged on a hexagonalgrid, the unit switch cells may be arranged on either a hexagonal orrectangular grid.

The embodiments disclosed herein use a two-dimensional array ofcapacitive micromachined ultrasound transducers (cMUTs) as theunderlying grid from which larger elements are constructed. The presentinvention is not limited, however, to cMUT structures and is equallyapplicable to other conventional or future transducer technologies.

One aspect of the invention is a device comprising: a multiplicity ofsensors arranged along a first set of substantially parallel lines in afirst stratum; a multiplicity of unit electronics cells arranged along asecond set of substantially parallel lines in a second stratum, thefirst and second sets of lines being substantially mutually parallel andaligned with each other; and a multiplicity of electrical connections,each of the electrical connections electrically connecting a respectiveone of the unit electronics cells to a respective one of the sensors,wherein each of the unit electronics cells comprises: a respectiveplurality of switches for closing respective pathways to a respectiveconnection point that is electrically connected to a respective sensorand that is not switchably disconnectable from the respective sensor;and a respective control circuit for controlling the switch states ofthe switches.

Another aspect of the invention is a device comprising a multiplicity ofsensors built in or on a first substrate; a multiplicity of unitelectronics cells integrated in a second substrate disposed adjacent toand confronting the first substrate; a multiplicity of bus linessupported by the second substrate; and a multiplicity of electricalconnections disposed between the first and second substrates, each ofthe electrical connections electrically connecting a respective one ofthe unit electronics cells to a respective one of the sensors, whereineach of the unit electronics cells comprises a respective plurality ofswitches and a respective control circuit for controlling the switchstates of the switches, each plurality of switches comprising an accessswitch that connects the associated sensor with one of the bus lineswhen the access switch is turned on, and a matrix switch that connects asensor associated with a neighboring unit electronics cell to the onebus line via the access switch when the access switch and the matrixswitch of the unit electronics cell are turned on.

A further aspect of the invention is a device comprising: a multiplicityof ultrasonic transducer subelements arranged in a first stratum, eachsubelement comprising a respective plurality of cMUT cells that areelectrically interconnected with each other and are not switchablydisconnectable from each other; a multiplicity of unit CMOS electronicscells arranged along a second stratum that underlies the first stratum;a multiplicity of bus lines; and a multiplicity of electricalconnections, each of the electrical connections electrically connectinga respective one of the unit CMOS electronics cells to a respective oneof the ultrasonic transducer subelements, wherein each of the unit CMOSelectronics cells comprises a respective plurality of switches and arespective control circuit for controlling the switch states of theswitches, each plurality of switches comprising an access switch thatconnects the associated ultrasonic transducer subelement with one of thebus lines when the access switch is turned on, and a matrix switch thatconnects an ultrasonic transducer subelement associated with aneighboring unit CMOS electronics cell to the one bus line via theaccess switch when the access switch and the matrix switch of the unitCMOS electronics cell are turned on.

Other aspects of the invention are disclosed and claimed below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a cross-sectional view of a typical cMUTcell.

FIG. 2 is a drawing showing a “daisy” subelement formed from sevenhexagonal MUT cells having their top and bottom electrodes respectivelyconnected together without intervening switches. This drawing is takenfrom U.S. patent application Ser. No. 10/383,990.

FIG. 3 is a drawing showing a sector of a mosaic array comprising fourannular elements as disclosed in U.S. patent application Ser. No.10/383,990, each element consisting of a tessellation of “daisy”subelements configured to have approximately equal area per element.

FIG. 4 is a drawing showing a cross-sectional view of a co-integratedcMUT and application specific integrated circuit (ASIC) array.

FIG. 5 is a drawing showing a cross-sectional view of a cMUT devicesubstrate connected to an ASIC switch matrix.

FIG. 6 is a drawing showing a top view of a hexagonal array of cMUTsubelements atop a hexagonal array of associated unit switch cells.

FIG. 7 is a drawing showing a top view of a hexagonal array of cMUTsubelements atop a rectangular array of associated unit switch cells.

FIG. 8 is a drawing showing translation of an annular transducer elementacross an array.

FIG. 9 is a drawing showing an architecture wherein all system channelsare distributed throughout the array such that each transducersubelement has access to every system channel.

FIG. 10 is a drawing showing an architecture wherein the number ofswitches in each subelement is limited by having one bus line per row ofsubelements, the bus lines being connected to system channels via amultiplexer.

FIG. 11 is a drawing showing an architecture having multiple bus linesper row of subelements, making it possible to group subelements ondifferent system channels within the same row.

FIG. 12 is a drawing showing an architecture in accordance with oneembodiment of the invention that allows a subelement in a first row toconnect to a bus line for a second row of subelements by connecting toan access switch of an adjacent subelement in the second row via amatrix switch of the subelement in the first row.

FIG. 13 is a drawing showing an architecture in accordance with anotherembodiment of the invention that allows a particular subelement in aparticular row of a cMUT array to be connected to any one of amultiplicity of system channel bus lines.

FIG. 14 is a drawing showing a hexagonal array of subelements with buslines connected to respective columns of subelements via access switches(indicated by solid dots).

FIG. 15 is a drawing showing a hexagonal array of subelements with somesubelements connected to vertical and horizontal bus lines viarespective access switches (indicated by solid dots).

FIG. 16 is a drawing showing a hexagonal array of subelements with buslines disposed diagonally along the natural axes of the hexagonal array.Access switches are indicated by solid dots.

FIG. 17 is a drawing showing connections to a common connection point inthe electronics associated with a particular acoustical subelement inaccordance with the embodiment depicted in FIG. 13.

FIG. 18 is a drawing showing components of a representative unit switchcell built beneath and electrically connected to an acousticalsubelement (not shown).

FIG. 19 is a drawing showing an access switch and circuitry forcontrolling the state of that access switch, as previously disclosed inU.S. patent application Ser. No. 10/248,968.

FIG. 20 is a drawing showing an arrangement of access and matrixswitches for use with rings (portions of which are indicated by dashedarcs) with single subelement width that are packed close together.Access switches are indicated by solid dots; matrix switches areindicated by dashes.

FIG. 21 is a drawing showing a representative cMUT cell with top andbottom electrodes connected to an electronics layer by metallized vias.

FIG. 22 is a drawing showing an embodiment of the invention whereinmetal routing between the sensor and electronics planes includesrerouting that allows use of an electronics chip that is larger than thesensor array.

Reference will now be made to the drawings in which similar elements indifferent drawings bear the same reference numerals.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to a reconfigurable sensor array having anintegrated switching matrix. For purposes of illustration, thereconfigurable array will be described with reference to capacitivemicromachined ultrasonic transducers (cMUTs). However, it should beunderstood that the aspects of the invention disclosed herein are notlimited in their application to probes employing cMUTs, but rather mayalso be applied to probes that employ pMUTs or even diced piezoceramicarrays where each of the diced subelements are connected by interconnectmeans to an underlying switching layer. The same aspects of theinvention also have application in reconfigurable arrays of optical,thermal or pressure sensors.

Referring to FIG. 1, a typical cMUT transducer cell 2 is shown in crosssection. An array of such cMUT transducer cells is typically fabricatedon a substrate 4, such as a heavily doped silicon (hence,semiconductive) wafer. For each cMUT transducer cell, a thin membrane ordiaphragm 8, which may be made of silicon nitride, is suspended abovethe substrate 4. The membrane 8 is supported on its periphery by aninsulating support 6, which may be made of silicon oxide or siliconnitride. The cavity 14 between the membrane 8 and the substrate 4 may beair- or gas-filled or wholly or partially evacuated. Typically, cMUTsare evacuated as completely as the processes allow. A film or layer ofconductive material, such as aluminum alloy or other suitable conductivematerial, forms an electrode 12 on the membrane 8, and another film orlayer made of conductive material forms an electrode 10 on the substrate4. Alternatively, the bottom electrode can be formed by appropriatedoping of the semiconductive substrate 4.

The two electrodes 10 and 12, separated by the cavity 14, form acapacitance. When an impinging acoustic signal causes the membrane 8 tovibrate, the variation in the capacitance can be detected usingassociated electronics (not shown in FIG. 1), thereby transducing theacoustic signal into an electrical signal. Conversely, an AC signalapplied to one of the electrodes will modulate the charge on theelectrode, which in turn causes a modulation in the capacitive forcebetween the electrodes, the latter causing the diaphragm to move andthereby transmit an acoustic signal.

The individual cells can have round, rectangular, hexagonal, or otherperipheral shapes. Hexagonal shapes provide dense packing of the cMUTcells of a transducer subelement. The cMUT cells can have differentdimensions so that the transducer subelement will have compositecharacteristics of the different cell sizes, giving the transducer abroadband characteristic.

Unfortunately, it is difficult to produce electronics that would allowindividual control over such small cells. While in terms of theacoustical performance of the array as whole, the small cell size isexcellent and leads to great flexibility, control is limited to largerstructures. Grouping together multiple cells and connecting themelectrically allows one to create a larger subelement, which can havethe individual control while maintaining the desired acousticalresponse. One can form rings or elements by connecting subelementstogether using a switching network. The elements can be reconfigured bychanging the state of the switching network. However, individualsubelements cannot be reconfigured to form different subelements.

MUT cells can be connected together (i.e., without intervening switches)in the micromachining process to form subelements. The term “acousticalsubelement” will be used in the following to describe such a cluster.These acoustical subelements will be interconnected by microelectronicswitches to form larger elements by placing such switches within thesilicon layer or on a different substrate situated directly adjacent tothe transducer array. This construction is based on semiconductorprocesses that can be done with low cost in high volume.

As used herein, the term “acoustical subelement” is a single cell or agroup of electrically connected cells that cannot be reconfigured, i.e.,the subelement is the smallest independently controlled acoustical unit.The term “subelement” means an acoustical subelement and its associatedintegrated electronics. An “element” is formed by connecting subelementstogether using a switching network. The elements can be reconfigured bychanging the state of the switching network. At least some of theswitches included in the switching network are part of the “associatedintegrated electronics”, as explained in greater detail below.

For the purpose of illustration, FIG. 2 shows a “daisy” transducersubelement 16 made up of seven hexagonal cMUT cells 2: a central cellsurrounded by a ring of six cells, each cell in the ring beingcontiguous with a respective side of the central cell and the adjoiningcells in the ring. The top electrodes 12 of each cMUT cell 2 areelectrically coupled together by connections that are not switchablydisconnectable. In the case of a hexagonal array, six conductors radiateoutward from the top electrode 12 and are respectively connected to thetop electrodes of the neighboring cMUT cells (except in the case ofcells on the periphery, which connect to three, not six, other cells).Similarly, the bottom electrodes 10 of each cell 2 are electricallycoupled together by connections that are not switchably disconnectable,forming a seven-times-larger capacitive transducer subelement 16.

Subelements of the type seen in FIG. 2 can be arranged to form atwo-dimensional array on a semiconductive (e.g., silicon) substrate.These subelements can be reconfigured to form elements, such as annularrings, using a switching network. Reconfigurability using silicon-basedultrasound transducer subelements was described in U.S. patentapplication Ser. No. 10/383,990. One form of reconfigurability is themosaic annular array, also described in that patent application. Themosaic annular array concept involves building annular elements bygrouping subelements together using a reconfigurable electronicswitching network. The goal is to reduce the number of beamformingchannels, while maintaining image quality and improving slice thickness.To reduce system channels, the mosaic annular array makes use of thefact that for an unsteered beam, the delay contours on the surface ofthe underlying two-dimensional transducer array are circular. In otherwords, the iso-delay curves are annuli about the center of the beam. Thecircular symmetry of the delays leads to the obvious grouping of thosesubelements with common delays. The reconfigurability can be used tostep the beam along the larger underlying two-dimensional transducerarray in order to form a scan or image.

There are numerous ways in which one can form transducer arrays usingMUT cells and acoustical subelements. FIG. 3 shows one example oftessellations of acoustical subelements to form a mosaic array. In theembodiment shown in FIG. 3, four approximately annular elements(referenced by numerals 18A-D respectively), each comprising atessellation of “daisy” acoustical subelements (seven MUT cellsconnected together per subelement), are configured to have approximatelyequal area per element. The tessellation in each case can be made up ofmultiple subelement types. The array pattern need not be a tessellation,but can have areas without acoustical subelements. For instance, therecould be vias to bring top electrode connections of the acousticalsubelement or cells below the array.

The configurations of the invention can be changed to optimize variousacoustic parameters such as beamwidth, side lobe level, or depth offocus. Alternatively, the acoustical subelements could be grouped toform one aperture for the transmit operation and immediately switched toanother aperture for the receive portion. While FIG. 3 shows respectiveportions of approximately annular elements, other configurations can beimplemented, for example, non-continuous rings, octal rings, or arcs.The choice of pattern will depend on the application needs.

Most apertures will consist of contiguous grouped subelementsinterconnected to form a single larger element, such as the annularelements shown in FIG. 3. In this case, it is not necessary to connectevery subelement directly to its respective bus line. It is sufficientto connect a limited number of subelements within a given group and thenconnect the remaining subelements to each other. In this way thetransmit signal is propagated from the system along the bus lines andinto the element along a limited number of access points. From there thesignal spreads within the element through local connections.

Given a particular geometry, the reconfigurable array maps acousticalsubelements to system channels. This mapping is designed to provideimproved performance. The mapping is done through a switching network,which is ideally placed directly in the substrate upon which the cMUTcells are constructed, but can also be in a different substrateintegrated adjacent to the transducer substrate. Since cMUT arrays arebuilt directly on top of a silicon substrate, the switching electronicscan be incorporated into that substrate. For a PZT or more conventionalimplementation, the switch network would simply be fabricated in aseparate silicon substrate and attached to the PZT array.

A cross-sectional view of a co-integrated cMUT and ASIC array is shownin FIG. 4 to illustrate how the connections would be made from the ASICto the cMUTs. As shown, a single via 56 is used to connect each cMUTsubelement 32 to its counterpart CMOS subelement (or “cell”) 50. Thevias 56, which connect the pads 65 of the signal electrodes torespective conductive pads 66 formed on the switch ASIC, may be embeddedin an acoustic backing layer 62 or other suitable insulating material.

FIG. 21 shows portions 50A and 50B of an electronics cell formed in asubstrate 90, which is separated from a cMUT subelement 32 by apassivation layer 92. Only one cMUT cell 2 of subelement 32 is shown,but it should be appreciated that each subelement comprises more thanone cMUT cell connected together in a manner that is not switchablydisconnectable. As shown in FIG. 21, it may be desirable to have morethan one signal per subelement. In particular, both the top electrode 12and the bottom electrode 10 can be brought down to the electronics cell,e.g., by means of metallized vias that pass through the passivationlayer 92. This provides independent control of both faces of the cMUTsubelement, which can be used to independently bias all cMUT subelementsin the array at different bias voltages. This feature could be used, forexample, to reverse the polarity of the transmit pulse or to adjust forslight variations in cMUT sensitivity from subelement to subelement.

It is also possible to build the cMUTs on a separate substrate (e.g., awafer) and connect them to the ASIC switch matrix separately, as shownin FIG. 5. Here for example, electrically conductive bumps 64 andelectrically conductive pads 65, 66 are used to connect the individualcMUT subelements 32 to their switch electronics counterparts 50. Otherinterconnect techniques such as anisotropic conductive paste (ACP),anisotropic conductive film (ACF), electrically conductive polymers,metallized bumps, vertical interconnect systems, e.g., z-axisinterposers, flexible printed circuits, etc. or metallized vias couldalso be used.

For optimum packing density it is useful to tile the cMUT subelements 32and the associated electronics on a hexagonal grid as illustrated inFIG. 6, which shows a top view of the ASIC switch matrix. Here the CMOSunit switch cells 50 are disposed in columns where every second columnis offset by half a cell height. With proper choice of the celldimensions, this will yield a perfect hexagonal array of pads 66 asshown. The vias 56 (also arranged in a hexagonal array) then connect tothe respective pads (not shown in FIG. 4) that form the basis ofconnections to the transducer layer above, comprising a hexagonal arrayof subelements. A more straightforward ASIC implementation isillustrated in FIG. 7. Here the CMOS unit switch cells 50 are arrangedin horizontal rows and vertical columns to form a rectangular grid,while the hexagonal subelements 32 above them form a hexagonal grid. Asshown in FIG. 7, the unit switch cell pads 66, arranged in rows andcolumns to form a rectangular array, still line up correctly to producethe connections such that the unit switch cells 50 are electricallyconnected to respective hexagonal subelements 32. In either case, thehexagonal grid pattern of the subelements makes it possible to realizethe mosaic annular array beam patterns as shown in FIG. 3.

In typical operation, the reconfigurable array is programmed with aninitial aperture pattern similar to the one shown in FIG. 3. Thispattern allows the beamformer to create a beam in front of the array.During imaging, the aperture is scanned across the array 60 asillustrated in FIG. 8, where the ring goes from ring 1 at t=1 to ring 2at t=2 and finally ring N at t=N, where t is time and N is a positiveinteger greater than 2. In this way the beam is swept in space in frontof the array and the beamformed echoes are used to build up successivelines of the image. The purpose of a reconfigurable array is to be ableto accomplish the imaging operation illustrated in FIG. 8 electronicallyfor an arbitrarily complex array pattern. Previous ultrasound scannersare capable of accomplishing electronic scanning but are limited in thecomplexity of the aperture due to lack of fine distribution of sensorsubelements in the elevation direction and fixed geometry.

A fully reconfigurable array as illustrated in FIG. 8 presents a numberof significant challenges in implementation. The sensor array issubdivided into tens of thousands of subelements. Beam patterns arebuilt up by grouping the subelements in their connections to a finitenumber of system transmit/receive and beamforming channels. When used toimplement the mosaic annular array concept, the reconfigurable arraywill form multiple rings that are translated across the arrayelectronically. At each new step in the translation, the entire ringpattern is reprogrammed into the array to create a new configuration.One could also provide the ability to update ring patterns betweentransmit and receive and at multiple intervals during receive to reducethe distortion of the beam as formed, thereby improving the imagequality.

In typical systems, 128 or more beamforming channels are used. Currentultrasound systems use multiplexing architectures that can route the 128system channels to a fixed number of transducer elements. Usingjudicious design of these multiplexer networks, it is possible to createa standard scanning pattern with a limited amount of electronics. Inmost cases however, the scanning pattern is fixed and not reconfigurabledue to the limitations of the network. A fully reconfigurable array doesnot suffer from these limitations; however, it requires a very denseswitching matrix to implement it.

As is illustrated in FIG. 8, the fundamental nature of thereconfigurable array requires that any subelement can be arbitrarilyconnected to any system channel. For example, as the aperture is scannedfrom the first location to the next location, the subelement S2 firstmust be part of an internal ring (not shown) and then must be part ofring 2. This means that it must switch from being connected to a firstsystem channel to being connected to a different system channel in ashort period of time. This is generally true of a large number ofsubelements in the array during scanning operation.

The simplest way to implement this requirement would be to distributeall system channels throughout the array such that each subelement hasaccess to every system channel. This architecture is illustrated in FIG.9. Here only five system channels are shown for illustration. Eachsystem channel is bussed through every subelement with local switchesused to select which system channel is picked up by which subelement.

In a system where the matrix electronics lie directly behind thetransducer array, the space for each subelement's switching electronicsis reduced to the size of the subelement. In typical ultrasound systemsthis size is on the order of a few hundred microns but could be smallerthan this. Since the size of a switch varies inversely with its onresistance, one is faced with a tradeoff: more switches with higher onresistance or fewer switches with lower on resistance. Even taking theextreme case however, in which the switches are as small as they can be,it soon becomes apparent that with present semiconductor technologies,many more than 16 switches cannot fit readily in the allotted space.Since for a real array the fully populated architecture of FIG. 9 willcontain still more switches, it appears to be intractable with thecurrent state of the art.

Although future technologies may make it quite feasible to integratemany more switches in the same space, progress in ultrasound will tendto reduce the allotted cell size since it is related to the wavelengthof the imager, which must shrink for improved image quality. Inaddition, many more components, such as digital control andtransmit/receive circuits, will migrate into this same limited area.Therefore, the fully populated architecture, while attractive for itssimplicity, is not immediately tenable or practicable.

A better solution to the interconnect problem described above is tolimit the number of switches in each subelement while at the same timeproviding for the flexibility required in a reconfigurable array. Thiscan be done by using a limited number of bus lines and making thesereconfigurable, as is illustrated in FIG. 10. Here a multiplexer 70 isused to arbitrarily select any of the system channels 38 (CH.1 throughCH. N) to be connected to any of the bus lines 74, with each row ofsubelements 32 served by only a single bus line. The cMUT cells 2 ofeach subelement (only one cMUT cell is shown for each subelement) areconnected to a bus line by means of a respective access switch 30. A keyfeature of this architecture is that many of the switches are locatedoutside of the array and therefore are not constrained by the geometryof the transducers. A one-dimensional pattern can be scanned across thearray using this architecture by successively selecting which row ofsubelements is connected to which system channel. A further improvementto this architecture is shown in FIG. 11. Here multiple bus lines 74, 76are routed down each row of subelements 32. The cMUT cells 2 of eachsubelement 32 can be connected either to bus line 74 via access switch30′ or to bus line 76 via access switch 30. This architecture providesflexibility in the horizontal direction since it is now possible togroup subelements on different system channels within the same row.

A further improvement to the above architecture can be made by realizingthat most apertures will consist of contiguous grouped subelementsinterconnected to form a single larger element. In this case, it is notnecessary to connect every subelement directly to its respective busline. It is sufficient to connect a limited number of subelements withina given group and then connect the remaining subelements to each other.In this way the transmit signal is propagated from the system along thebus lines and into the element along a limited number of access points.From there the signal spreads within the element through localconnections. This architecture is illustrated in FIG. 12. Hereindividual subelements 32 are able to connect to the bus line associatedwith their row by way of access switches 30 and are able to connect tothe bus line associated with an adjacent row by way of matrix switches36, which connect one subelement to an adjacent subelement.

One embodiment of the invention, shown in FIG. 13, incorporates all ofthe above-mentioned improvements together. Here an access switch 30 isused to connect a given subelement 32 to a row bus line of bus 34. Thisarchitecture is directly applicable to a mosaic annular array. In such adevice multiple rings can be formed using the present architecture,wherein each ring is connected to a single system channel using one ormore access switches, each of which is connected to a bus line, which isin turn connected to a system channel.

The access switches are staggered as shown in FIG. 13 to reduce thenumber required for a given number of bus lines, as discussed furtherbelow. Random ordering of access switches to bus lines (not shown) couldalso be employed to reduce artifacts due to the repeating patterns. Morethan one access switch in each subelement could be used to improve theflexibility of the array. In such an architecture, a tradeoff betweenflexibility and number of access switches per subelement would be madewhere the number is still significantly fewer than the number of buslines and system channels. It is also possible to use more than oneaccess switch per bus line in each element. This would improve the yieldof the device since non-functioning access switches could be bypassedusing the redundant access switches.

The row bus lines are connected to the system channels using across-point switching matrix as shown in FIG. 13. A sparse cross-pointswitch could be used as well in which fewer multiplexer switches wouldbe required. Such an architecture would be more efficient in use ofspace but would require judicious choice of switch configurations toensure that all bus lines could be properly connected. As shown in FIG.12, multiple bus lines can be used per row. More bus lines improvesflexibility of the array at the expense of more multiplexer switches andmore routing area inside the array. It is possible to skip rows or touse different numbers of bus lines on different rows. For example, toconserve area it might be advantageous to share a group of bus linesbetween every pair of adjacent rows of subelements.

Although only horizontal bus lines have been discussed thus far, it isalso possible to dispose both vertically and horizontally running buslines within an array. Bus lines could be disposed vertically asillustrated in FIG. 14 (see bus lines 72, 74, 76). Referring to FIG. 15,one set of bus lines 82 could be disposed horizontally and another set(only one bus line 84 is shown) disposed vertically. In this case eachsubelement or group of subelements will be connectable to a vertical busline via one access switch and will further be connectable to ahorizontal bus line via a different access switch. However, in the casewhere bus lines are run in both directions because the electronic realestate available for bus lines is running low and more bus lines areneeded, but there is still only a single access switch in a subelement,then each subelement's access switch could be connected to either thehorizontal bus line or the vertical bus line and not both. Finally, buslines could also be disposed diagonally as illustrated in FIG. 16. Theselines 76, 80 respectively run along two of the natural axes of thehexagonal array and would therefore simplify addressing of subelements.

The number of access switches and row bus lines is determined by thesize constraints and the application. For the purpose of disclosing oneexemplary non-limiting implementation (shown in FIG. 13), a singleaccess switch 30 for each subelement 32 and four row bus lines 34 a-34 dfor each row of the array will be assumed. The second type of switch isa matrix switch 36, which is used to connect a connection point 42 ofone subelement (see FIG. 17) to the connection point of a neighboringsubelement. This allows an acoustical subelement 32 to be connected to asystem channel through the integrated electronics associated with aneighboring acoustical subelement. This also means that an acousticalsubelement may be connected to a system channel even though it is notdirectly connected via an access switch. While FIG. 13 shows threematrix switches 36 per subelement, it is also possible to have fewerthan three to conserve area or to allow for switches which have lower onresistance and therefore have larger area. In addition, matrix switchescan be used to route around a known bad subelement for a given array.Finally, while hexagonal subelements are shown, rectangular subelementsare also possible and these might require fewer switches.

Referring to FIG. 17, each of the subelements comprises a commonconnection point 42 in the electronics associated with the acousticalsubelement 32. This common connection point 42 electrically connectseight components in each subelement. The common connection point 42connects the acoustic subelement or transducer 32 to the access switch30 for that subelement, to the three matrix switches 36 associated withthat subelement, and to the three matrix switches associated with threeneighboring subelements via connections 46. A signal that travelsthrough a matrix switch gets connected to the common connection point ofthe neighboring subelement.

FIG. 13 depicts how the switching network might work for a particularsubelement. This is only an exemplary arrangement. A bus 34, whichcontains four row bus lines 34 a through 34 d, runs down the row ofsubelements 32. FIG. 13 shows only three subelements in this row, but itshould be understood that other subelements in this row are not shown.The row bus lines of bus 34 are multiplexed to system channel bus linesof system channel bus 38 at the end of a row by means of multiplexingswitches 40, which form a cross-point switching matrix. As seen in FIG.13, each row bus line 34 a-34 d can be connected to any one of thesystem channel bus lines of bus 38 by turning on the appropriatemultiplexing switch 40 and turning off the multiplexing switches thatconnect the particular row bus line to the other system channel buslines. These multiplexing electronics can be off to the side and thusare not as restricted by size. FIG. 13 shows a fully populatedcross-point switch. However, in cases wherein it is not necessary tohave switches that allow every bus line to be connected to every systemchannel, a sparse cross-point switch can be used in which only a smallsubset of the system channels can be connected to a given bus line, inwhich case only some of switches 40 depicted in FIG. 13 would bepresent.

An access switch is so named because it gives a subelement direct accessto a bus line. In the exemplary implementation depicted in FIG. 13,there are six other switch connections for each subelement. Theseconnections take the form of matrix switches 36. A matrix switch allowsa subelement to be connected to a neighboring subelement. While thereare six connections to neighboring subelements for each subelement inthis hexagonal pattern, only three switches reside in each subelementwhile the other three connections are controlled by switches in theneighboring subelements. Thus there is a total of four switches andassociated digital addressing and control logic (not shown) in eachsubelement. This is just one exemplary implementation. The number of buslines, the number of access switches, and the number and topology of thematrix switches could all be different, but the general concept wouldremain.

FIG. 18 shows some of the components of a representative unit switchcell built beneath and electrically connected (via connection point 42)to an acoustical subelement (not shown). The unit switch cell may beelectrically coupled to the acoustical subelement via a metal pad 66 ofthe type depicted in FIG. 4. The unit switch cell comprises an accessswitch 30 that connects the connection point 42 to a bus line 34 andthree matrix switches 36. These switches are of a type that have switchstate memory for storing the current switch state. The unit switch cellfurther comprises latches 88 (only one of which is shown) for storingdata representing the future switch states of the access switch 30 andthe three matrix switches 36. The latches are standard CMOS memoryelements, however, other memory elements such as EPROM, EEPROM, MRAM orMEMS could also be used. The future switch state data is received via adigital data bus 45 comprising multiple bus lines (only one bus linebeing shown in FIG. 18). In response to a write signal received via acontrol bus 44 comprising multiple bus lines (again only one bus line isshown), the future switch state data on data bus 45 is written into thelatches 88. In response to a read signal received via the control bus 44during a subsequent cycle, the switch state data is read out from thelatches and converted (by logic not shown) into control signals thatwill change the states of the switches accordingly. These new switchstates will be stored in the switch state memories of the switches. Thelatch 88 and switches 30 and 36 receive voltage supplies via power line90.

Although the access and matrix switches can be separately packagedcomponents, it is possible to fabricate the switches within the samesemiconductor substrate on which the MUT array is to be fabricated. Theaccess and matrix switches may comprise high-voltage switching circuitsof the type disclosed in U.S. patent application Ser. No. 10/248,968entitled “Integrated High-Voltage Switching Circuit for UltrasoundTransducer Array”. As seen in FIG. 19, each switch (e.g., an accessswitch 30) comprises two DMOS FETs that are connected back to back(source nodes shorted together) to allow for bipolar operation. Currentflows through the switch terminals whenever both FETs are turned on. Thestates of the switches are controlled by a respective switch controlcircuit 52. The states of the switch control circuits are in turndictated by outputs from a programming circuit 54, which programs theswitch control circuits in accordance with a desired switchingconfiguration. The programming circuit may be implemented using a viewgenerator and address and data generator circuits of the types disclosedin U.S. patent application Ser. No. ______ filed concurrently herewithand entitled “Method and Apparatus for Controlling Scanning of MosaicTransducer Array”, the disclosure of which is fully incorporated byreference herein. The switch control circuits may also be implemented inaccordance with one of the embodiments disclosed in the latter patentapplication. The switches could be CMOS, DMOS, BiCMOS, BCDMOS, MEMS orany other highly integrated switching technology available currently orin the future.

FIG. 19 shows an acoustical subelement 32 connected to an access switch30 via a common connection point 42. The six other lines that connect tothe connection point 42 are not shown. For this example, the accessswitch 30 comprises the aforementioned pair of back-to-back DMOS FETs.The control circuit 52 turns the switch 30 on or off as a function ofswitch state data signals sent by the programming circuit 54. Whenaccess switch 30 is turned on, the acoustical subelement 32 (e.g., asubarray of interconnected cMUT cells) is connected to a row bus line 34a. For this configuration, the electronics associated with eachacoustical subelement (i.e., the “unit switch cell”) will comprise oneaccess switch, three matrix switches, a respective control circuit foreach of these four switches, and a respective conductor connecting thecommon connection point to the matrix switches of three neighboringsubelements (not shown). Optionally, each unit switch cell alsocomprises latches for storing the future switch states of the switchesin that unit switch cell, as disclosed in U.S. patent application Ser.No. ______ filed concurrently herewith. The addition of digital memoryin the form of latches is useful in that it implements the requirementfor fast transition of aperture patterns between successive transmit andreceive operations.

Still referring to FIG. 19, the signal that travels from the acousticalsubelement 32 to the row bus line 34 a is the electrical receive signal.Here the receive signal is the electrical response generated by theacoustical subelement 32 when a sound pressure wave interacts with thetransducer. The transmit signal, in which an electrical pulse isgenerated by the ultrasound system, travels from the row bus line 34 ato the acoustical subelement 32. For a given channel, this electricalexcitation pulse travels through a system channel bus line to a row busline. The signal travels from the row bus line to the acousticalsubelement through an access switch 30 and also travels to othersubelements through the matrix switches (not shown in FIG. 19).

The number of switches that fit behind an acoustical subelement islimited. The size of the switch determines the on resistance of theswitch and the smaller the switch the larger the on resistance. Thedelay and distortion caused by the switching increases as the switch onresistance increases. This means that there is a tradeoff between thenumber of switches behind an acoustical subelement and the delayintroduced by those switches. One solution to that tradeoff involvesreducing the number of switches to a small number while retaining asmuch flexibility as possible. This reduction is achieved by using matrixswitches to allow acoustic subelements to be attached to a systemchannel through other subelements, and by limiting the number of accessswitches to a small number.

The bus lines that connect the access switches to the systems channelsalso take space in the electronics layer, so minimizing the number ofbus lines is also beneficial. The number of unique channels that can bedirectly connected to acoustic subelements in the same row is determinedby the number of bus lines. However, since the matrix switches allowsubelements in one row to connect to subelements in other rows, thenumber of channels in a row is increased by the matrix switches. Thisallows the number of bus lines to be kept small, while still servicing alarge number of channels. Of course, having more bus lines increases theflexibility but requires more space.

The use of matrix switches means that the number of access switchesbehind each subelement can be reduced. In the extreme case there is onlyone access switch for each subelement. However, if there is more thanone bus line, a determination must be made as to which bus line eachaccess switch should be connected. One solution is to stagger theconnections so that the bus line connected to repeats every Nsubelements in a row, where N is a number determined by the requirementof minimum signal distortion as discussed below. Returning to FIG. 13,each subelement 32 in the row is connected to one of the row bus linesin the row bus 34 via a respective access switch 30. This pattern ofstaggered connections repeats every four subelements. The staggeringallows more bus lines with fewer access switches and combined with thematrix switches, still allows for great flexibility as to which systemchannels can be connected to each subelement. Of course having more thanone access switch per cell increases the flexibility of the connectionsbut requires smaller switches with higher on resistance.

Generally, the number of rows N after which the pattern repeats isdetermined by the maximum number of matrix switches which can be strungtogether while still maintaining adequate signal integrity. This numbercomes out of the understanding that the matrix switch resistance andcMUT capacitances together form an RC delay line with a time constant ofdelay which varies exponentially with the number of series taps N.Staggering the access switches on multiple row bus lines allows thenumber of elements that can be supported to be increased given theconstraint of the delay line. As illustrated in FIG. 20, the worst casefor the design occurs where rings (portions of which are indicated bydashed arcs) with single subelement width are packed close together. Thevertical sections of the ring provide the worst case since bus lines 74,76 in this design run horizontally. In the horizontal sections of therings, one could just use a single access switch at every subelementsince they would all be the same as the bus lines run parallel to therings. In the vertical sections however, every row of subelements 32 isassociated with a different bus line that is connected to a differentsystem channel. Therefore, subelements spaced vertically in this areacan only be supported using matrix switches 36, represented by dashes.In FIG. 20, there are two bus lines per row, and the pattern of accessswitches 30 (represented by dots) repeats every four rows subelements.At each row, two rings are supported by the two access switches andtheir associated string of subelements grouped with matrix switches.Since the pattern repeats after four rows, this particular architecturewill support a maximum of 2×4=8 rings. In general for an array with Mbus lines on each row and N taps for each string of subelements, amaximum of K system channels can be supported where K=M×N. Of course,most sections of the rings will be neither perfectly horizontal norperfectly vertical. Therefore the task of the system designer is tooptimize the array configuration at all points in the aperture under theconstraints of the architecture. Various methods for optimizing such aswitching configuration are disclosed in U.S. patent application Ser.No. ______ filed concurrently herewith and entitled “Optimized SwitchingConfigurations for Reconfigurable Transducer Arrays”.

In accordance with a further aspect of the invention, the metal routingbetween the sensor and electronics planes includes rerouting that allowsuse of an electronics chip or chips having an area larger than the areaof the sensor array, as seen in FIG. 22. FIG. 22 depicts a plurality oftransducer subelements 32 built on a substrate 94, with a pair ofelectronics chips 90A and 90B laminated to the bottom of the substrate94, each chip comprising a respective plurality of unit switch cells 50.The metal routing 96 diverges, with the connections to the sensor planebeing confined to an area smaller than the area of the electronics chips90A and 90B.

While the invention has been described with reference to preferredembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationto the teachings of the invention without departing from the essentialscope thereof. Therefore it is intended that the invention not belimited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A device comprising a multiplicity of sensors arranged along a firstset of substantially parallel lines in a first stratum; a multiplicityof unit electronics cells arranged along a second set of substantiallyparallel lines in a second stratum; and a multiplicity of electricalconnections, each of said electrical connections electrically connectinga respective one of said unit electronics cells to a respective one ofsaid sensors, wherein each of said unit electronics cells comprises: arespective plurality of switches for closing respective pathways to arespective connection point that is electrically connected to arespective sensor and that is not switchably disconnectable from saidrespective sensor; and control circuitry for controlling the switchstates of said switches.
 2. The device as recited in claim 1, whereineach of said electrical connections comprises a respective bump made ofelectrically conductive material, anisotropic conductive paste (ACP),anisotropic conductive film (ACF), an electrically conductive polymer, ametallized bump, a vertical interconnect system, e.g., a z-axisinterposer, a flexible printed circuit, or a metallized via.
 3. Thedevice as recited in claim 1, further comprising a layer of materialdisposed between said first and second strata, wherein each of saidelectrical connections comprises a respective metallized via in saidlayer of material.
 4. The device as recited in claim 1, furthercomprising a multiplicity of bus lines, wherein at least one of saidswitches in each unit electronics cell is an access switch that connectsthe associated sensor with one of said bus lines when said access switchis turned on.
 5. The device as recited in claim 4, wherein at least oneof said switches in each unit electronics cell is a matrix switch thatconnects the sensor associated with a neighboring unit electronics cellto said one bus line via said access switch when said access switch andsaid matrix switch of each unit electronics cell are turned on.
 6. Thedevice as recited in claim 1, wherein even-numbered lines of saidsensors are offset relative to odd-numbered lines of said sensors anddistributed with the correct spacing to form a hexagonal grid ofsensors.
 7. The device as recited in claim 6, wherein even-numberedlines of said unit electronics cells are offset relative to odd-numberedlines of said electronics cells to form a hexagonal grid of unitelectronics cells that generally matches said hexagonal grid of saidsensors, and said multiplicity of electrical connections are alsoarranged in a hexagonal array.
 8. The device as recited in claim 6,wherein said unit electronics cells are arranged in horizontal rows andvertical columns to form a rectangular grid of unit electronics cells,and said multiplicity of electrical connections are also arranged in arectangular array with horizontal and vertical spacing that aligns withthe hexagonal grid of sensors.
 9. The device as recited in claim 1,wherein each of said sensors comprises a respective ultrasonictransducer subelement.
 10. The device as recited in claim 9, whereineach of said ultrasonic transducer subelements comprises a respectivemultiplicity of capacitive micromachined ultrasonic transducer cellsthat are interconnected to each other and are not switchablydisconnectable from each other.
 11. The device as recited in claim 1,wherein each of said unit electronics cells comprises a respectiveplurality of memory devices for storing future switch states of saidswitches.
 12. The device as recited in claim 1, wherein saidmultiplicity of sensors occupies about the same area as saidmultiplicity of unit electronics cells.
 13. The device as recited inclaim 1, wherein said multiplicity of sensors and said multiplicity ofunit electronics cells are co-integrated on the same substrate.
 14. Thedevice as recited in claim 1, wherein said multiplicity of sensors aremicromachined in or on a first substrate, and said multiplicity of unitelectronics cells are integrated on a second substrate, said first andsecond substrates being arranged to form a stack.
 15. The device asrecited in claim 1, wherein said switches are CMOS switches.
 16. Thedevice as recited in claim 1, wherein said sensors are arranged in ahexagonal grid, and each of said unit electronics cells comprises threematrix switches for connecting each unit electronics cell to threeadjacent unit electronics cells.
 17. The device as recited in claim 1,wherein said sensors are arranged in a hexagonal grid, and each of saidunit electronics cells comprises a respective pad made of electricallyconductive material, each pad being electrically connected to arespective one of said electrical connections, said pads being arrangedin a rectangular array.
 18. A device comprising a multiplicity ofsensors built in or on a first substrate; a multiplicity of unitelectronics cells integrated in a second substrate disposed adjacent toand confronting said first substrate; a multiplicity of bus linessupported by said second substrate; and a multiplicity of electricalconnections disposed between said first and second substrates, each ofsaid electrical connections electrically connecting a respective one ofsaid unit electronics cells to a respective one of said sensors, whereineach of said unit electronics cells comprises a respective plurality ofswitches and a respective control circuit for controlling the switchstates of said switches, each plurality of switches comprising an accessswitch that connects the associated sensor with one of said bus lineswhen said access switch is turned on, and a matrix switch that connectsa sensor associated with a neighboring unit electronics cell to said onebus line via said access switch when said access switch and said matrixswitch of the unit electronics cell are turned on.
 19. The device asrecited in claim 18, wherein each of said electrical connectionscomprises a respective bump made of electrically conductive material,said bumps being disposed between said first and second substrates. 20.The device as recited in claim 18, wherein each of said sensorscomprises a respective ultrasonic transducer subelement.
 21. The deviceas recited in claim 20, wherein each of said ultrasonic transducersubelements comprises a respective multiplicity of capacitivemicromachined ultrasonic transducer cells that are interconnected toeach other and are not switchably disconnectable from each other. 22.The device as recited in claim 18, further comprising a layer ofmaterial disposed between said first and second substrates, wherein eachof said electrical connections comprises a respective metallized via insaid layer of material.
 23. The device as recited in claim 22, whereinsaid material of said layer is acoustic backing material.
 24. The deviceas recited in claim 6, wherein said sensors are arranged in a hexagonalgrid and said unit electronics cells are arranged in a hexagonal gridthat generally matches said hexagonal grid of said sensors.
 25. Thedevice as recited in claim 6, wherein said sensors are arranged in ahexagonal grid and said unit electronics cells are arranged in arectangular grid.
 26. A device comprising: a multiplicity of ultrasonictransducer subelements arranged in a first stratum, each subelementcomprising a respective plurality of cMUT cells that are electricallyinterconnected with each other and are not switchably disconnectablefrom each other; a multiplicity of unit CMOS electronics cells arrangedalong a second stratum that underlies said first stratum; a multiplicityof bus lines; and a multiplicity of electrical connections, each of saidelectrical connections electrically connecting a respective one of saidunit CMOS electronics cells to a respective one of said ultrasonictransducer subelements, wherein each of said unit CMOS electronics cellscomprises a respective plurality of switches and a respective controlcircuit for controlling the switch states of said switches, eachplurality of switches comprising an access switch that connects theassociated ultrasonic transducer subelement with one of said bus lineswhen said access switch is turned on, and a matrix switch that connectsan ultrasonic transducer subelement associated with a neighboring unitCMOS electronics cell to said one bus line via said access switch whensaid access switch and said matrix switch of the unit CMOS electronicscell are turned on.
 27. The device as recited in claim 26, wherein saidultrasonic transducer subelements are arranged in a hexagonal grid andsaid unit CMOS electronics cells are arranged in a rectangular grid. 28.The device as recited in claim 26, further comprising a layer ofmaterial disposed between said ultrasonic transducer subelements andsaid unit CMOS electronics cells, wherein each of said electricalconnections comprises a respective metallized via in said layer ofmaterial.
 29. The device as recited in claim 28, wherein said materialof said layer is acoustic backing material.
 30. The device as recited inclaim 10, wherein each of said capacitive micromachined ultrasonictransducer cells comprises a top electrode and a bottom electrodeconnected to a respective unit electronics cell via respective vias. 31.The device as recited in claim 1, wherein said multiplicity of sensorsoccupies an area less than the area occupied by said multiplicity ofunit electronics cells.
 32. The device as recited in claim 21, whereineach of said capacitive micromachined ultrasonic transducer cellscomprises a top electrode and a bottom electrode connected to arespective unit electronics cell via respective vias.
 33. The device asrecited in claim 18, wherein said first substrate has an area less thanthe area of said second substrate.